Optical imaging onto a moving substrate is well known, for example in laser printers and photocopiers, which employ a photoconductive drum for receiving the optical image. Normally optical information, such as an image, is imaged or written directly onto a photoconductive drum using appropriate stationary optics together with a rotating polygon or hologon or a galvano-mirror to scan the drum axially. Examples of such systems are illustrated and described in U.S. Pat. Nos. 4,796,961; 4,547,038; 4,445,125 and 4,474,422.
It has been found by the applicants that potentially troublesome distortions can occur due to uneven velocity of the photoconductive drum during imaging. This is particularly true in multi-color, multi-pass photocopying wherein registration misalignment may result. Additionally, image distortions during the writing of the image may be different for different color images. The varying distortions result in local areas of misregistration between images. These problems are especially severe for multi-beam systems.
Methods for at least partially correcting some of these problems are known such as changing the timing of the light signals, as described in U.S. Pat. No. 4,445,128.